Rotary drill bits are often used to drill wellbores. One type of rotary drill bit is a fixed-cutter drill bit that has a bit body comprising matrix and reinforcement materials, i.e., a “matrix drill bit” as referred to herein. Matrix drill bits are typically manufactured by placing powder material into a mold and infiltrating the powder material with a binder material, such as a metallic alloy. The various features of the resulting matrix drill bit, such as blades, cutter pockets, and/or fluid-flow passageways, may be provided by shaping the mold cavity and/or by positioning temporary displacement materials within interior portions of the mold cavity. A preformed bit blank (or steel mandrel) may be placed within the mold cavity to provide reinforcement for the matrix bit body and to allow attachment of the resulting matrix drill bit with a drill string. A quantity of matrix reinforcement material (typically in powder form) may then be placed within the mold cavity with a quantity of the binder material.
The mold is then placed within a furnace and heated to a desired temperature to allow the binder (e.g., metallic alloy) to liquefy and infiltrate the matrix reinforcement material. The furnace typically maintains a desired temperature until the infiltration process is deemed complete, such as when a specific location in the bit reaches a certain temperature. Once the designated process time or temperature has been reached, the mold is then removed from the furnace and begins to rapidly lose heat to its surrounding environment via heat transfer, such as radiation and/or convection in all directions.
This heat loss continues to a large extent until the mold is moved and placed on a cooling or quench plate and an insulation enclosure or “hot hat” is lowered around the mold. The insulation enclosure drastically reduces the rate of heat loss from the top and sides of the mold while heat is drawn from the bottom of the mold through the cooling plate. This controlled cooling of the mold and the infiltrated matrix bit contained therein can facilitate axial solidification dominating radial solidification, which is loosely termed directional solidification. As the molten material of the infiltrated matrix bit cools, there is a tendency for shrinkage that could result in voids forming within the bit body unless the molten material is able to continuously backfill such voids. In some cases, for instance, one or more intermediate regions within the bit body may solidify prior to adjacent regions and thereby stop the flow of molten material to locations where shrinkage porosity is developing. In other cases, shrinkage porosity may result in poor metallurgical bonding at the interface between the bit blank and the molten materials, which can result in the formation of cracks within the bit body that can be difficult or impossible to inspect.
While the mold is positioned on the quench plate, water is often ejected out of one or more nozzles provided in the quench plate to impinge upon the bottom of the mold and thereby promote directional solidification. As it contacts the heated mold, however, the water can generate a significant amount of steam or vapor that often enters the insulation enclosure and increases heat transfer from the upper section of the mold, possibly by wetting the insulation (thereby increasing its conductivity) or by creating or enhancing convective currents inside the insulation enclosure. This additional cooling can produce multiple solidification fronts, which result in blank bond-line cracking, apex cracking, binder-rich zones, bevel cracking, and cracking between nozzles.